The present invention relates generally to a method for increasing the efficiency of a vapor compression system by removing heat in the compressor from the system with the heat accepted by the heat sink of the heat rejecting heat exchanger.
Chlorine containing refrigerants have been phased out in most of the world due to their ozone destroying potential. Hydrofluoro carbons (HFCs) have been used as replacement refrigerants, but these refrigerants still have high global warming potential. xe2x80x9cNaturalxe2x80x9d refrigerants, such as carbon dioxide and propane, have been proposed as replacement fluids. Unfortunately, there are problems with the use of many of these fluids as well. Carbon dioxide has a low critical point, which causes most air conditioning systems utilizing carbon dioxide to run transcritical, or above the critical point.
When a vapor compression system runs transcritical, the high side pressure of the refrigerant is typically high so that the refrigerant does not change phases from vapor to liquid while passing through the heat rejecting heat exchanger. Therefore, the heat rejecting heat exchanger operates as a gas cooler in a transcritical cycle, rather than as a condenser. The pressure of a subcritical fluid is a function of temperature under saturated conditions (where both liquid and vapor are present). However, the pressure of a transcritical fluid is a function of fluid density when the temperature is higher than the critical temperature.
In a prior vapor compression system, the heat generated by the compressor motor either is lost by being discharged to the ambient or superheats the suction gas in the compressor. If the heat is lost to the ambient, it is not transferred usefully, reducing system efficiency. Alternatively, if the heat superheats the suction gas in the compressor, the density and the mass flow rate of the refrigerant decrease, also decreasing system efficiency.
Another prior system has employed a tapping circuit which branches off from the heat sink of the heat rejecting heat exchanger to cool the compressor motor. After the cooling fluid in the tapping circuit accepts heat from the compressor motor, the tapping circuit returns to flow of the heat sink of the heat rejecting heat exchanger. A drawback to this system is that the cooling fluid which accepts heat from the compressor motor returns to the heat sink heated, lessening the ability of the cooling fluid to accept additional heat from the heat rejecting heat exchanger.
Two-stage compression systems employing an intercooler positioned between the compression stages has also been utilized to increase system efficiency. In a prior system, the refrigerant in the intercooler exchanges heat with the ambient or with a circuit of cooling fluid separate from the circuit of cooling fluid in the heat sink of the heat rejecting heat exchanger.
Efficiency of a vapor compression system is increased by usefully transferring heat in the compressor from the system with the heat accepted by the heat sink of the heat rejecting heat exchanger. In one embodiment, a stream of cooling fluid absorbs heat from the compressor motor. Preferably, the cooling fluid is water. The heated stream of cooling fluid merges with the heated fluid medium exiting the heat sink of the gas cooler and exits the system. The efficiency of the system is equal to the useful heat transferred divided by the work put into the cycle. As the heat of the compressor is usefully transferred out of the system rather than being lost to the ambient, system efficiency increases. Additionally, by removing the heat in the compressor motor, superheating of the suction gas in the compressor is reduced, increasing the density and mass flow rate of the refrigerant to further increase efficiency.
Alternatively, heat from the compressor motor is transferred to a secondary heat exchange medium, such as oil. The heated oil then transfers heat into the stream of cooling fluid for removal from the system.
In another embodiment, an intercooler is employed between compression stages for compressor cooling. After the fluid medium absorbs heat from the refrigerant in the gas cooler, the heated fluid medium travels to the intercooler to accept additional heat from the refrigerant in the intercooler. The heated fluid medium then usefully exits the system. As the heat in the intercooler is usefully transferred out of the system and is not lost, system efficiency is increased. Additionally, as the refrigerant exiting the intercooler is cooled, the mass flow rate and density of the refrigerant in the second stage of compression is increased, also increasing efficiency.
These and other features of the present invention will be best understood from the following specification and drawings.